Multilayer coating

Abstract

The present invention relates to a multilayer coating obtained by performing the following steps: (1) applying a ZnNi layer to a substrate material, in particular steel, (2) performing a first heat treatment in a temperature range of 135-300 °C, preferably 185-220 °C, for a period of at least 4 hours, preferably at least 23 hours, (3) applying a metal-pigmented topcoat to the ZnNi layer, and (4) performing a second heat treatment in a temperature range of 150-250 °C, preferably 180-200 °C, for a period of at least 10 minutes, preferably at least 20 minutes, preferably at least 30 minutes.

Description

[0001] The present invention relates to a multilayer coating and a manufacturing method for this multilayer coating.

[0002] Hydrogen re-embrittlement of coated steel can lead to a significant deterioration in the steel's properties. The deposition of individual hydrogen atoms in the steel's lattice structure causes an increase in pressure within the lattice matrix and leads to a detrimental change in the steel's brittleness. This can subsequently lead to hydrogen-induced cracking of the steel, which can result in the steel no longer maintaining its normally assigned strength properties. Such cracking would have devastating consequences, particularly in the aviation sector, where key components such as the main landing gear, the transmission, or bolts are made of steel.

[0003] Re-embrittlement of coated high-strength steel typically occurs as a result of corrosion. This involves the formation of atomic hydrogen on the surface of the high-strength steel and / or the coating, either through hydrogen corrosion or another chemical reaction. Some of the atomic hydrogen diffuses into the material (=steel) before combining to form a non-diffusible H2 molecule. The atomic hydrogen that has diffused into the material accumulates at lattice defects in the steel's lattice structure and leads to embrittlement of the steel. The resulting increase in pressure inside the steel can lead to brittle cracking or fracture.

[0004] The disclosures of US 7,514,153 B1 and EP 1 683 891 A1 show a multilayer coating which has the features of the generic term.

[0005] EP 0 508 306 A1 discloses a method for the corrosion protection coating of steel workpieces, in which the workpiece surface is copper-plated and galvanized before being provided with a further layer containing metallic components and organic binders and the further layer is hardened by heat treatment.

[0006] The object of the present invention is to create a multilayer coating that can be applied to high-strength steel and exhibits particularly pronounced corrosion resistance and the associated resistance to hydrogen re-embrittlement. Thus, components coated with the multilayer coating according to the invention can be used for an extended period of time without being impaired by potentially possible hydrogen re-embrittlement.

[0007] This object is achieved by the multilayer coating on a component of an aircraft, in particular a main landing gear, a push rod, a gear, or a bolt, according to claim 1. The multilayer coating according to the invention is thus obtained by implementing the features listed in the claim. It is preferable that the steps outlined above for obtaining the multilayer coating be carried out in the specified order.

[0008] According to the invention, a low-hydrogen embrittlement ZnNi layer is used as the ZnNi layer applied to the substrate. A high-strength steel such as 300M or AISI 4340 is used as the substrate material for the ZnNi layer.

[0009] Preferably, the metal-pigmented topcoat(s) comprise(s) an organic or an inorganic matrix or binding matrix, with an organic binding matrix being preferred.

[0010] After the heat treatment, which also serves to degas any hydrogen atoms generated during the application of the ZnNi layer to the substrate, one (or more) metal-pigmented topcoats are applied to the ZnNi layer. The metal-pigmented topcoat(s) consist of a mixture of zinc and aluminum flakes bonded by an inorganic or organic matrix. The layer can be applied by spray application or a dip-spin coating process. The metal-pigmented topcoat is preferably applied at room temperature, at an air humidity of 30 to 80% rel.H.

[0011] A second heat treatment is then performed to bake in the metal-pigmented topcoat. A convection oven can be used, controlled so that the surface temperature of the topcoat is in the range of 180°C to 200°C. The heat treatment lasts at least 30 minutes.

[0012] The result is a multilayer coating that offers outstanding corrosion protection. The metal-pigmented topcoat is porous and permeable, making the multilayer coating electrochemically active. This maintains cathodic protection. Furthermore, the corrosion current i corr is preferably less than 5 µA / cm 2 .

[0013] The adhesion strength of the multilayer coating when applied to a high-strength steel is greater than 4N / mm 2< in a pull-off test and shows only isolated cohesive failure.

[0014] In addition, the multilayer coating reduces the corrosion rate of the ZnNi layer. This also results in a reduction in the corrosion rate compared to an uncoated or overpainted version of a ZnNi layer applied to a substrate. A particularly advantageous feature is that the multilayer coating virtually eliminates hydrogen embrittlement under corrosion stress. Thus, its resistance to corrosion-related, hydrogen-induced damage is better than simply coating a substrate material, such as steel or high-strength steel, with a low-temperature ZnNi coating.

[0015] The thickness of the LHE-ZnNi layer of the multilayer coating according to the invention is preferably at most 30 µm, preferably at most 20 µm. The organic content is less than 100 milligrams per liter.

[0016] It is also advantageous if the dry film thickness of the metal-pigmented topcoat(s) is at most 10 µm, preferably at most 7 µm, and most preferably at most 5 µm. The dry film thickness of the topcoat is determined after the second heat treatment.

[0017] A further optional feature of the present invention is that, prior to applying an LHE-ZnNi layer to a substrate, the substrate is blasted with an intensity of at most 0.1 mm Almen A. The substrate can be blasted, for example, with white corundum F180 (=EKF180).

[0018] Almen intensity measurement provides a means of comparing different blasting processes. It determines the deformation caused by the blasting process on a defined sample. The statement 0.1 mm Almen A indicates that a type A test strip with a thickness of 1.29 mm was used for intensity measurement. If this strip is subjected to the blasting process, it exhibits a bending deformation of 0.1 mm at its saturation point (doubling the blasting duration results in only a 10% increase in deflection). Since measurements using Almen intensity measurement are well known in the art, this process will not be discussed in further detail. The effect of blasting is to free the substrate material or steel from any contaminants.

[0019] Furthermore, it is possible to passivate the LHE-ZnNi layer before applying the metal-pigmented topcoat. The passivation can be performed before or after the initial heat treatment and can be carried out with or without chromium (VI).

[0020] Furthermore, after the application of the metal-pigmented topcoat and before carrying out the second heat treatment, a flash-off time of at least 5 minutes, preferably at least 10 minutes, more preferably at least 20 minutes, may be provided.

[0021] Furthermore, the present invention relates to a method for producing a multilayer coating on a high-strength steel having the features listed in claim 7.

[0022] The metal-pigmented topcoat(s) preferably comprise an organic or inorganic matrix or binding matrix, with an organic binding matrix being preferred.

[0023] Preferably, before applying an LHE-ZnNi layer to the substrate, the substrate is blasted with an intensity of at most 0.1 mm Almen A. This leads to cleaning of the substrate, so that a deposition of an LHE-ZnNi layer can be carried out effectively on the substrate.

[0024] In addition, the LHE-ZnNi layer can be passivated before or after the initial heat treatment. The passivation can be performed with or without chromium (VI).

[0025] Preferably, after application of the metal-pigmented topcoat, a flash-off time of at least 5 minutes, preferably at least 10 minutes, more preferably at least 20 minutes is provided.

[0026] The invention is explained in more detail below with reference to the accompanying figures. Many of the figures relate to a series of tests conducted to demonstrate the advantages of the multilayer coating according to the invention. They show: Fig. 1 shows a comparison of the corrosion resistance of the coating according to the invention compared to conventional coatings, Fig. 2a-c shows a creep rupture diagram, an enlarged image of a fracture surface and an SEM image of a fracture surface of a steel coated with the multilayer coating according to the invention, Fig. 3a-e shows a creep rupture diagram, an enlarged image of a fracture surface and two SEM images of a fracture surface of a steel coated with the multilayer coating according to the invention and Fig. 4a-c shows a creep rupture diagram, an enlarged image of a fracture surface and an SEM image of a fracture surface of a steel coated with a conventional ZnNi layer.

[0027] The stripe in the middle shows a steel that has only been coated with a ZnNi layer and the two right-hand stripes show a steel that has only been coated with a metal-pigmented topcoat but has no ZnNi layer.

[0028] The Fig. 1 The coatings shown were all subjected to an identical corrosion test, so that the corrosion resistance of the various coatings can now be assessed based on a visual inspection. It can be seen at first glance that the multilayer coating according to the invention has a much better corrosion resistance than the coatings also shown in Fig.1The coatings shown are non-inventive. Almost no traces of corrosion are visible. In particular, the two right-hand strips, which do not have a zinc-nickel coating, are very severely corroded. The centrally located strip of steel coated with a zinc-nickel coating shows a somewhat less advanced state of corrosion.

[0029] This result is confirmed because in a reembrittlement test according to NAVAL Warfare (45% / 24h+5% / 1h), the behavior of the multilayer coating according to the invention on a steel, in particular a high-strength steel, against corrosion-related, hydrogen-induced damage is better than a steel with an LHE-ZnNi layer.

[0030] The overall impression that the multilayer coating according to the invention is superior to a conventional ZnNi layer is also confirmed by the series of tests presented below.

[0031] Incremental step-load tests are performed under media exposure in a 3.5% NaCl solution at room temperature on notched tensile specimens made of material 300M with different coating variants according to ASTM F519. Following the test, a fracture surface analysis is performed to determine operational hydrogen-induced damage due to corrosion exposure (reembrittlement tests).

[0032] The test series includes the following samples: • 2 sets of 4 notched tensile specimens, number: la, in the condition: LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h +TC P35 (5 µm) + WBH 190°C / 30 min (according to the invention) • 2 sets of 4 notched tensile specimens, number: 1b, in the condition: LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h +TC P35 (10 µm) + WBH 190°C / 30 min (according to the invention) • 1 set of 4 notched tensile specimens, number: 1c, in the condition: LHE ZnNi (LU) + passivation + WBH 190°C / 23 h (comparative example)

[0033] The orienting reembrittlement tests are carried out according to the test procedure specified below. Test procedure for Reembrittfement tests:

[0034] Table 1: Test procedure for reembrittlement tests Pre-existing stress Passed embrittlement test Incremental step load 45% F mK for 24 hours Subsequent hourly increase by 5% F mK Test duration Max. 24 + 10 hours temperature Room temperature (20 ± 3°C) Test medium 175 ± 2 ml 3.5% NaCl solution, pH 6.9 ± 0.1, not purged with nitrogen, naturally aerated la - LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h +TC P35 (5 µm) + WBH 190°C / 30 min

[0035] In this sample, two sets of four notched tensile specimens are created. The zinc-nickel layer used is a LHE ZnNi (LLI) with passivation, which is subjected to an initial heat treatment at 190°C for 23 hours. A topcoat (=coating) of type P35 from Magni is used as a metal-pigmented topcoat. This is applied with a thickness of 5 µm. A second heat treatment is then carried out to bake in the metal-pigmented topcoat. The second heat treatment at 190°C lasts 30 minutes. Briefly, this can be summarized as follows: LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h + TC P35 (5 µm) + WBH 190°C / 30 min.

[0036] The material used as the carrier material for the multilayer coating is a steel of type 300M, batch 065 / Z (F mk = 42960 N).

[0037] The test parameters are as follows: 45% F mK 24 h + 5% F mK 1 h each; max. 24 + 10 hours; test medium 3.5% NaCl, pH 7; test bench: Zwick Z050. Table 2: Test results on sample Ia Results Sentence sample Downtime in hours max. force in % F mK / N Result (> 50% passed) remark 1 1 33:00:36 90 / 40500 passed - 2 33:01:12 90 / 40500 passed - 3 33:01:48 90 / 40500 passed - 4 33:02:24 90 / 40500 passed - 2 1 33:00:36 90 / 40800 passed - 2 33:01:12 90 / 40800 passed - 3 33:01:48 90 / 40800 passed - 4 33:01:48 90 / 40800 passed -

[0038] The creep diagrams of one of the samples 1-4 of set 1 and set 2 are shown in Fig. 2a specified.

[0039] In addition, a microscopic representation of the fracture surfaces of each sample is shown in Fig. 2b . shown.

[0040] Fig. 2c shows 4 images of a sample in which enlarged images of the layer thicknesses in the notch root of the notch tensile specimen and outside the notch are visible.

[0041] The incremental step load tests show that both sets of four specimens withstood a test load of 90% F mK. The shortest test duration for both sets was 33 h 36 s. The metallographic analyses of variant 1a show that the ZnNi coating is present throughout the specimen. The average layer thickness of the metal-pigmented topcoat (ZnL layer) is 16.0 µm. The layer thickness of the topcoat is approximately 33.0 µm ( Fig. 2c , upper images). In the notch root, the ZnNi coating has a thickness of 8.5 µm and the topcoat has a thickness of 20.5 µm ( Fig. 2c , lower images).

[0042] The metallographic analyses of variant 1a show that the ZnNi coating is present throughout the sample examined. The average layer thickness of the ZnNi coating is 16.0 µm. The layer thickness of the top coat is approximately 33.0 µm ( Fig. 2c, upper illustrations). In the notch root, the ZnNi coating has a thickness of 8.5 µm and the TopCoat coating has a thickness of 20.5 µm ( Fig. 2c , lower images). Ib - LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h + TC P35 (10 µm) + WBH 190°C / 30 min

[0043] In this sample, two sets of four notched tensile specimens are created. The zinc-nickel layer used is an LHE ZnNi (LLI) with passivation, which is subjected to an initial heat treatment at 190°C for 23 hours. A topcoat (TC) of type P35 from Magni is used as a metal-pigmented topcoat. This is applied with a thickness of 10 µm. A second heat treatment is then carried out, one of the purposes of which is to bake in the metal-pigmented topcoat. The second heat treatment at 190°C lasts 30 minutes. In brief, this can be summarized as follows: LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h + TC P35 (10 µm) + WBH 190°C / 30 min.

[0044] The material used as the carrier material for the multilayer coating is a steel of type 300M, batch 065 / Z (F mk = 42960 N).

[0045] The test parameters are as follows: 45% F mK 24 h + 5% F mK 1 h each; max. 24 + 10 hours; test medium 3.5% NaCl, pH 7; test bench: Zwick Z050. Table 3: Test results of sample Ib Results Sentence sample Downtime in hours max. force in % F mK / N Result (> 50% passed) remark 1 1 33:00:36 90 / 40600 passed - 2 33:01:12 90 / 40600 passed - 3 33:01:48 90 / 40600 passed - 4 33:02:24 90 / 40600 passed - 2 1 26:08:24 55 / 25800 passed - 2 26:09:00 55 / 25800 passed - 3 26:09:42 55 / 25800 passed - 4 26:10:30 55 / 25800 passed -

[0046] The creep diagrams of one of the samples 1-4 of set 1 and set 2 are shown in Fig. 2a specified.

[0047] In addition, a microscopic representation of the fracture surfaces of each sample is shown in Fig. 2b . shown.

[0048] The incremental step load tests show that the two sets of specimens tested withstood significantly different test loads. The four specimens of set 1 withstood a test load of 90% F mK, while those of set 2 withstood only 55% F mK. The shortest test duration for set 1 was 33 h 36 s and for set 2, 26 h 8 min 24 s.

[0049] The metallographic analyses of sample 1, set 1 show that the ZnNi coating has an average thickness of 13.3 µm and the topcoat coating has a thickness of 29.0 µm ( Fig. 3c , upper images). On sample 1, set 2, the layer thicknesses are on average 16.3 µm for ZnNi and 24.0 µm for TopCoat ( Fig. 3d , upper illustrations).

[0050] The investigations further show that the ZnNi coating on the examined sets 1 and 2 is of varying thickness in the area of ​​the notch root. Thus, sample 1, set 1, has both a ZnNi coating with a thickness of 5.0 µm and a topcoat coating of 12.5 µm ( Fig. 3c , lower images). Sample 1, set 2, however, shows only traces of the ZnNi coating ( Fig. 3d , lower images). Furthermore, the topcoat coating in this sample is not continuous down to the notch root.

[0051] The scanning electron microscopic fracture surface analysis of sample 1 of the second set shows that it shows significant damage due to hydrogen embrittlement after the incremental step load test ( Fig. 3e , lower illustration). The upper right illustration of the Fig. 3e shows the topcoat coating in the area of ​​the notch base.

[0052] The metallographic analyses of sample 1, set 1 show that the ZnNi coating has an average thickness of 13.3 µm and the topcoat coating has a thickness of 29.0 µm ( Fig. 3c , upper images). On sample 1, set 2, the layer thicknesses are on average 16.3 µm for ZnNi and 24.0 µm for TopCoat ( Fig. 3d , upper illustrations).

[0053] The investigations further show that the ZnNi coating on the examined sets 1 and 2 is of varying thickness in the area of ​​the notch root. Thus, sample 1, set 1, has both a ZnNi coating with a thickness of 5.0 µm and a topcoat coating of 12.5 µm ( Fig. 3c , lower images). Sample 1, set 2, however, shows only traces of the ZnNi coating ( Fig. 3e , lower illustrations). Furthermore, the topcoat coating is not continuous all the way to the notch base.

[0054] The scanning electron microscopic fracture surface analysis of sample 1 of the second set shows that it shows damage due to hydrogen embrittlement after the incremental step load test ( Fig. 3e , lower illustration). The upper illustrations of the Fig. 3e show the topcoat coating in the area of ​​the notch base. Ic - LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h

[0055] In this series of tests, a set of 4 notched tensile specimens is prepared, whereby the coating applied to the steel here is not the multilayer coating according to the invention.

[0056] The coating used for the 300M steel grade is a zinc-nickel layer, specifically a layer of LHE ZnNi (LLI), with passivation, which is subjected to heat treatment at 190°C for 23 hours. Briefly, this can be summarized as follows: LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h.

[0057] No additional coating is applied to the ZnNi layer, nor is a second heat treatment performed.

[0058] The material used as the carrier material for the coating is steel type 300M, batch 065 / Z (F mk = 42960 N).

[0059] The test parameters are as follows: 45% F mK 24 h + 5% F mK 1 h each; max. 24 + 10 hours; test medium 3.5% NaCl, pH 7; test bench: Zwick Z050. Table 4: Test results on sample Ic Results Sentence sample Downtime in hours max. force in % F mK / N Result (> 50% passed) remark 1 1 26:10:12 55 / 25800 passed - 2 26:15:00 55 / 25800 passed - 3 26:16:12 55 / 25800 passed - 4 26:17:24 55 / 25800 passed -

[0060] The creep diagram of sample 1-4 is shown in Fig. 4a specified.

[0061] In addition, a microscopic representation of the fracture surfaces of each sample is shown in Fig. 4b . shown.

[0062] The incremental step load tests show that the tested specimens withstood a test load of 55% F mK. The shortest test duration was 26 h 10 min 12 s.

[0063] Metallographic analysis shows that the ZnNi coating is present right down to the notch root. The average layer thickness of the ZnNi coating is 9.0 µm and 6.5 µm at the notch root ( Fig. 4c ).

[0064] Table 5 below provides a summary of the results. Table 5: Tabular summary of the results of the reembrittlement test Sample designation Condition Sentence Nr. max. force in % F mK / N Test duration h:min:s Result (> 50% passed) Ia LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h + TC P35 (5 µm) + WBH 190°C / 30 min 1 1 90 / 40500 33:00:36 passed 2 90 / 40500 33:01:12 passed 3 90 / 40500 33:01:48 passed 4 90 / 40500 33:02:24 passed 2 1 90 / 40800 33:00:36 passed 2 90 / 40800 33:01:12 passed 3 90 / 40800 33:01:48 passed 4 90 / 40800 33:01:48 passed Ib LHE ZnNi (LLI) + passivation + WBH 180°C / 23 h + TC P35 (10 µm) + WBH 190°C / 30 min 1 1 90 / 40600 33:00:36 passed 2 90 / 40600 33:01:12 passed 3 90 / 40600 33:01:48 passed 4 90 / 40600 33:02:24 passed 2 1 55 / 25800 26:08:24 passed 2 55 / 25800 26:09:00 passed 3 55 / 25800 26:09:42 passed 4 55 / 25800 26:10:30 passed Ic LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h 1 1 55 / 25800 26:10:12 passed 2 55 / 25800 26:15:00 passed 3 55 / 25800 26:16:12 passed 4 55 / 25800 26:17:24 passed IIe LHE ZnNi (LLI) + passivation + WBH 190°C / 23 h 1 1 55 / 25800 26:07:48 passed 2 55 / 25800 26:09:36 passed 3 55 / 25800 26:17:24 passed + WBH according to LHT4-4103 4 55 / 25800 26:27:00 passed

[0065] From Table 5 it can be deduced that the multilayer coating according to the invention has an excellent corrosion resistance which is superior to the comparison samples, provided that the layer arrangement is continuous and does not have a defect near the notch root of the notch tensile test specimen as in sample Ib, set 2.

[0066] Below is a tabular overview of the layer thicknesses of the different samples. Table 6: Tabular summary of the layer thickness determination Sample description Sentence Nr. Layer thickness in µm comment notch base lateral surface ZnNi TopCoat ZnNi TopCoat Mw* Mw* Mw** Mw** Ia 2 2 8,5 20,5 16,0 33,0 - Ib 1 1 5,0 12,5 13,3 29,0 - Ib 2 1 0,0 0,0 16,3 24,0 Hardly any ZnNi and no topcoat detectable in the notch base Ic 1 1 6,5 0,0 9,0 0,0 - IIe 1 1 7,5 0,0 18,7 47,0 *Mean of two measured values ​​**Mean of three measured values

[0067] It can therefore be seen that the presence of a multilayer coating according to the invention significantly improves corrosion resistance compared to conventional coatings. This is due to the reduced hydrogen embrittlement resulting from the multilayer coating according to the invention.

Claims

1. Component in an aircraft which is provided with a multilayer coating obtained by carrying out the steps of: (1) applying a Low Hydrogen Embrittlement ZnNi layer, i.e. LHE ZnNi layer, to a high-strength steel, (2) carrying out a first heat treatment in a temperature range from 185-220°C for a time period of at least 23 hours, and (3) applying a metal-pigmented top coat to the LHE ZnNi layer, characterized by (4) carrying out a second heat treatment in a temperature range from 180 - 200°C for a time period of at least 30 minutes, wherein the steps are carried out in the stated order (1), (2), (3), (4), and the top coat consists of a mix of zinc and aluminum lamellae interconnected by an inorganic or organic matrix.

2. Component of claim 1, wherein the thickness of the LHE ZnNi layer is at most 30 µm, preferably at most 20 µm.

3. Component according to any one of the preceding claims, wherein the dry layer thickness of the metal-pigmented top coat in accordance with step (4) is at most 10 µm.

4. Component according to any one of the preceding claims, wherein, before step (1), a step is carried out for shot peening the substrate material with an intensity of at most 0.1 mm Almen A.

5. Component according to the preceding claim, wherein between step (2) and step (3), or between step (1) and step (2), a step is carried out for passivating the LHE ZnNi layer with or without chromium(IV).

6. Component according to any one of the preceding claims, wherein between step (3) and step (4), a step is carried out for venting the applied metal-pigmented top coat for at least 5 minutes, preferably at least 10 minutes, preferably at least 20 minutes.

7. Method of manufacturing a multilayer coating on a high-strength steel, comprising the steps of: (1) applying an LHE ZnNi layer, i.e. LHE ZnNi layer, to the high-strength steel, (2) carrying out a first heat treatment in a temperature range from 185-220°C for a time period of at least 23 hours, and (3) applying a metal-pigmented top coat to the LHE ZnNi layer, characterized by (4) carrying out a second heat treatment in a temperature range from 180 - 200°C for a time period of at least 30 minutes, wherein the steps are carried out in the order (1), (2), (3), (4), and the top coat consists of a mix of zinc and aluminum lamellae interconnected by an inorganic or organic matrix.

8. Method of claim 7, wherein the substrate material is shot-peened with an intensity of at most 0.1 mm Almen A before step (1).

9. Method of claim 7 or 8, wherein between step (2) and step (3), or between step (1) and step (2), the LHE ZnNi layer is passivated with or without chromium (IV).

10. Method of any one of claims 7 to 9, wherein a step is carried out between step (3) and step (4) to vent the applied metal-pigmented top coat for at least 5 minutes.

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